In general, a liquid crystal display is constructed of a backlight device and a liquid crystal panel. The backlight device includes a sheet metal back chassis and a front chassis, a light reflecting plate, a light source support portion, a light source, a light diffusing plate, an optional light conductive plate, and a light source drive circuit such as an inverter. The liquid crystal display has such a structure that the liquid crystal panel is mounted and fixed onto the backlight device. The backlight devices are roughly classified into three types: a directly-below type, a light conductive type, and a tandem type which is a hybrid of the former two. Since high luminance is required for the backlight device for use in a large-screen liquid crystal television out of the above backlight devices, the development of directly-below type and tandem (hybrid) type backlight devices (refer to Patent Document 1) is now vigorously underway.
The directly-below type backlight device of the conventional art includes a flat or wavy light reflecting plate which is formed by a resin foamed product being adhered and laminated onto an aluminum sheet metal substrate, a plurality of light sources, light source supports, a light diffusing plate, a plurality of optical films, and a housing structure composed of sheet metal housings (back chassis and front chassis) (refer to Patent Documents 2 to 4, for example).
The liquid crystal display of the conventional art is constructed by mounting the liquid crystal panel onto the above backlight device.
As for the light reflecting plate in use, a light reflecting plate manufactured by adhering and laminating a resin foamed product onto an aluminum sheet metal substrate in order to retain the warp, deformation, and structure of the reflecting plate, is used. In general, a reflector is manufactured by sheet metal processing such as pressing for forming a wavy form or bending for forming side faces.
As for the light sources, a plurality of light sources are used according to the display screen size of the liquid crystal display and luminance required for the backlight device. Linear or U-shaped cold-cathode tubes (CCFL), point light sources such as optical semiconductor devices (LED), or those which are arranged in a linear or planar form are used.
As for the light source supports, not only sheet metal, but also injection molded products of a thermoplastic resin composition are often used. Particularly, light source supports molded out of a polycarbonate resin composition containing titanium oxide have a light reflection function, and light source supports having a rib structure as well as a light source supporting function are used to improve the torsional stiffness of the light reflecting plate.
The light diffusing plate is generally made of an acrylic resin, a copolymer resin of an acrylic monomer and styrene, a polycarbonate resin, or, nowadays, a resin composition including a transparent resin such as cyclic olefin resin with a light diffusing agent blended therein. The light diffusing plate has a thickness of about 1 to 3 mm and is selected according to the size of a liquid crystal display screen and a lighting device system.
Besides, as the light diffusing plate support frame, a frame having not only light source support portions are used, but also having rib structures obtained by an injection molding a polycarbonate-based resin composition containing titanium oxide are employed.
As for the optical films, optical films having a plurality of functions are laminated. Light diffusing films used to make the surface luminance of a backlight device uniform and prism sheets having a luminance improving function are generally used. A plurality of the light diffusing films and a plurality of the prism sheets are laminated in order to control luminance and luminance uniformity.
Those backlight devices of the conventional art have a large number of parts and a large number of assembly steps.
In addition, sheet metal processing is carried out twice: sheet metal processing for manufacturing a reflector by the processing of a light reflecting plate into a wavy plate and bending for forming side faces; and sheet metal processing for manufacturing a chassis (housing), i.e. a housing structure, thereby taking time and labor and making it impossible to eliminate an increase in the weight of the whole device. In a light reflecting plate including a resin foamed material adhered and laminated onto an aluminum sheet metal substrate, the resin foamed layer easily comes off from the aluminum substrate and displaces during sheet metal processing, thereby making it difficult to carry out the sheet metal processing of a complex shape. The aluminum sheet metal substrate used herein is made of aluminum or aluminum alloy. To impart sheet metal processability to the aluminum, the 52S aluminum material which is generally available at relatively cheap price cannot be used, and an expensive material must be used. Since the sheet metal processing of a complex shape is difficult, light source supports having the functions of: supporting a reinforcing structure for preventing the light reflecting plate from twisting; supporting light sources; supporting reflection portions; and insulating heat generated from the terminals of light source electrodes must be manufactured by injection molding a resin composition separately, mounted and fixed after the light sources are installed on the light reflecting plate to construct a reflector. The thickness of the chassis forming the skeleton of the housing structure in the case of aluminum sheet metal is 1 mm for a display screen size of 22 inches, 1.5 mm for a display screen size of 30 inches, and 2 mm for a display screen size of 40 inches. The fact is also a cause for increase in the weight of the device (refer to Patent Documents 5 and 6, for example).
There is further proposed a sheet metal substrate manufactured by applying a coating composition having light reflection ability to a sheet metal substrate for a light reflecting plate. However, the sheet metal substrate is only used in a light reflecting plate, and a reflector having a structure that the sheet metal substrate serves as a back chassis and/or light source supports is not proposed. Light source supports manufactured by injection molding separately must be used in a light source support portion as described above.
Meanwhile, when a light reflecting plate is formed from a polycarbonate-based thermoplastic resin composition containing titanium oxide and having a light reflecting function without using the aluminum sheet metal substrate, it is difficult to suppress warp and deformation caused by thermal expansion due to a temperature rise by heat from the light sources.
To form a chassis for supporting a liquid crystal panel as required, it is difficult to ensure stiffness (refer to Patent Documents 7 and 8, for example).
Although there are proposed methods of improving the structures of light source electrode terminals which are the sources of heat to increase heat radiation, all of the methods cannot achieve in reducing the number of parts (refer to Patent Documents 9 to 11, for example).
Those backlight devices of the conventional art have a large number of parts and a large number of assembly steps. The backlight devices include a housing structure in which an iron or aluminum sheet metal chassis is installed on the back of a reflector and increase the weight of a liquid crystal display.
The reflecting material of the conventional art is a thin (190 μm) film formed by stretching a polyester and a polypropylene-based material, or a polyester-based foamed sheet (1 mm) formed by super critical fluid foaming, and it is difficult to provide a shape by heat processing while the original properties are maintained. Therefore, the thin film and an aluminum sheet metal are assembled and pressed together whereas the super critical foamed sheet is bent by incision. However, those steps are costly and take a lot of time and labor, and there are differences between shape design values estimated from optical properties and actual measurement values.
[Patent Document 1]
Japanese Patent Application Laid-Open (kokai) No. 2002-72204
[Patent Document 2]
Japanese Patent Application Laid-Open (kokai) No. 2004-22352
[Patent Document 3]
Japanese Patent Application Laid-Open (kokai) No. 2004-127643
[Patent Document 4]
Japanese Patent Application Laid-Open (kokai) No. 2001-215497
[Patent Document 5]
Japanese Patent Application Laid-Open (kokai) No. 2004-55182
[Patent Document 6]
Japanese Patent Application Laid-Open (kokai) No. 2004-139871
[Patent Document 7]
Japanese Patent Application Laid-Open (kokai) No. 2004-102119
[Patent Document 8]
Japanese Patent Application Laid-Open (kokai) No. 2003-162901
[Patent Document 9]
Japanese Patent Application Laid-Open (kokai) No. 2004-134281
[Patent Document 10]
Japanese Patent Application Laid-Open (kokai) No. 2001-216807
[Patent Document 11]
Japanese Patent Application Laid-Open (kokai) No. 2003-234012